Re: [PATCH v2] perf/x86: Move event pointer setup earlier in x86_pmu_enable()

[Peter Zijlstra]

On Wed, Mar 11, 2026 at 06:35:09PM +0100, Peter Zijlstra wrote:
> > Additionally, does this change leave the unthrottled event's hardware
> > counter uninitialized?
> 
> Also yes.

Something like so on top of things I suppose.

---
Subject: x86/perf: Make sure to program the counter value for stopped events on migration
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed Mar 11 21:29:14 CET 2026

Both Mi Dapeng and Ian Rogers noted that not everything that sets HES_STOPPED
is required to EF_UPDATE. Specifically the 'step 1' loop of rescheduling
explicitly does EF_UPDATE to ensure the counter value is read.

However, then 'step 2' simply leaves the new counter uninitialized when
HES_STOPPED, even though, as noted above, the thing that stopped them might not
be aware it needs to EF_RELOAD -- since it didn't EF_UPDATE on stop.

One such location that is affected is throttling, throttle does pmu->stop(, 0);
and unthrottle does pmu->start(, 0); possibly restarting an uninitialized counter.

Fixes: a4eaf7f14675 ("perf: Rework the PMU methods")
Reported-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
Reported-by: Ian Rogers <irogers@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/events/core.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1374,8 +1374,10 @@ static void x86_pmu_enable(struct pmu *p
 
 			cpuc->events[hwc->idx] = event;
 
-			if (hwc->state & PERF_HES_ARCH)
+			if (hwc->state & PERF_HES_ARCH) {
+				static_call(x86_pmu_set_period)(event);
 				continue;
+			}
 
 			/*
 			 * if cpuc->enabled = 0, then no wrmsr as